On the way to becoming Arkansas’ Number One grass weed, barnyardgrass has tormented farmers while picking up resistance to a wide range of herbicides. And without agronomic changes the weed is likely to ratchet up the pressure on Mid-South crops.

Propanil resistance in Arkansas barnyardgrass was documented in the early 1990s. Shortly thereafter, Facet (Quinclorac) came in under a Section 18 and was widely adopted. By the late 1990s, weed scientists began to find Quinclorac resistance.

“What was interesting is that when we found Quinclorac resistance in barnyardgrass, it was also resistant to Propanil,” says Jason Norsworthy, University of Arkansas weed scientist.

The march towards resistance to multiple herbicides continued.

In the early 2000s, growers began to use Command (Clomazone). By 2007, the state had its first case of Clomazone-resistant barnyardgrass.

In 2009, Norsworthy confirmed the first case of ALS chemistry-resistant barnyardgrass -- products like Regiment, Grasp, Beyond, and Newpath in Clearfield rice.

“Today, I’ve found seven populations within the state that are resistant to ALS herbicides. Two populations are resistant to Clomazone. We have numerous populations resistant to Propanil and Facet.

“Slightly more than 50 percent of the barnyardgrass samples that are sent to me for resistance testing are confirmed resistant to Propanil. In that same circumstance, 29 percent of barnyardgrass samples are resistant to Quinclorac.

“It’s also worth noting that 22 percent of all this barnyardgrass is resistant to both Facet and Propanil. So, many of our rice growers have completely lost two herbicide options.”

At the end of each growing season, growers, Extension agents and consultants send Norsworthy barnyardgrass samples for screening. “It’s intriguing that some of the fields haven’t had Facet or Propanil in them for almost a decade, now. However, they’re still resistant.

“So, once we lose a herbicide to resistance, it’s lost for the long haul. You can take it out of the system for a long time and it still won’t be effective when used again. Resistance in the barnyardgrass continues to persist.”

The resistance genes aren’t flushed out of the plant.

“What happens is there is no ‘fitness penalty’ for the plant having the resistance. As a result, once it goes into the soil seed bank, any offspring from the resistant barnyardgrass continues to carry the trait.”

And it isn’t just barnyardgrass.

“We’re seeing this with pigweed in cotton and soybeans. We’re seeing it with other weeds, as well.

“We’ve lost glyphosate on pigweed, something growers are well aware of. The same concept is playing out now in rice with barnyardgrass.”

The news isn’t good for the future, either.

“We still haven’t found a location where there’s Propanil, Facet, Command, and ALS resistance in the same field. But I think we’re quickly approaching the day when it will happen. Once it does, our growers will have very few options for controlling barnyardgrass in rice.”

Thankfully, there are other herbicides available.

Prowl, Bolero, RiceStar and Clincher – the latter two being ACCase, or Group 1, herbicides – “are basically the only other tools we’d have to manage barnyardgrass if we have a population that develops resistance to the four herbicides I mentioned earlier.

“When that happens it will be extremely challenging for growers to deal with barnyardgrass. It’s pretty easy for me to control it in small-plot research. I can easily flush a field, spray a herbicide in a timely manner and optimize its activity.”

But try putting Prowl, Bolero, RiceStar and Clincher across a large rice operation.

“You start looking at 1,000 or 2,000 acres of rice and it can take several days, if not a week, to get water across the fields much less deal with herbicides. That is not going to be a happy situation.”

Mid-South modeling

That fear is largely the impetus behind new management research by Norsworthy and colleagues. “We want to know what to do to minimize the risk of resistance evolving in our current rice production systems. What can we do with our chemical strategies and non-chemical strategies to reduce that risk of resistance? What’s the value of crop rotation versus going with continuous rice? What is the value of flooding at four- to five-leaf rice rather than at five- or six-leaf rice?

“What impact does it have if you make your pre-flood application and it takes seven days to get water across the field compared to two days? What does that mean for selection for herbicide resistance in barnyardgrass?

“What about keeping fields flooded during winter months versus not flooding? Does that have any impact on loss of seed from the soil seed bank and risk of developing resistance?”

The researchers are also looking at planting dates. What about moving planting dates up versus waiting until later?

“We did some computer modeling work with Palmer amaranth. If you look at the current herbicide recommendations for Palmer amaranth, especially in cotton, to a large extent they came out of that modeling work.”

When Norsworthy and fellow weed scientist Ken Smith (now working in Texas for Cheminova) first sat down and looked at Palmer amaranth, “we had the idea that we needed to load up the weed control system on the back end. In other words, we thought the plants that were escaping at lay-by in cotton were contributing most to resistance.”

In reality, the duo found that “a very, very strong residual herbicide needed to be down at planting. And we needed to overlay those residual herbicides throughout the season to be successful.

“So, we want to take that same model structure and move it over to barnyardgrass. We’re doing a tremendous amount of data collection; a lot goes into the model: the emergence pattern of barnyardgrass, how many seed are being produced from escapes – 20 or 30 factors.”

The effort has expanded beyond Arkansas borders. Norsworthy is now also working with weed scientists Daniel Stephenson and Jason Bond in Louisiana and Mississippi, respectively.

“This really goes beyond rice if we’re talking the Mid-South. My goal, in the next year or two, is for us to put together something similar to the ‘Pigposium’ (a very successful pigweed-specific meeting held in 2011) but highlighting barnyardgrass. This is the Number One grass weed we deal with in cotton and soybean.”

Norsworthy wants to provide growers with barnyardgrass solutions as quickly as possible but he says the research is rather open-ended.

“When can you say you have all the answers? Various funding sources have supported the research. The Rice Promotion Board has been very generous, especially around resistance management. The Southern IPM has recently funded a two-year project looking at non-chemical approaches to resistance management.

“But we want to get this modeling done. The idea is that it will help us prioritize and show us to what extent each practice is having an impact on resistance. And when you integrate several practices, to what extent does that reduce the risk of resistance?”